This invention relates to signal processing apparatus, and particularly to such apparatus as used in systems for simulating graphic arts process. The apparatus of the invention is particularly useful in connection with a signal transforming apparatus such as a video signal generator which transforms light signals into electrical signals.
Video signal generating apparatus is most commonly used in connection with television systems. In such systems, the light intensity of elementary portions of an image are sequentially sampled, usually in a raster pattern. The sequence of samples are joined to form a composite video signal having portions containing video or light intensity information and portions containing control information. The control information is usually between adjacent lines of light intensity information in a signal portion called the blanking interval. This portion of the signal corresponds to the time during which the electron beam of the television receiver picture tube is deflected back to start the next line. In this blanking interval, there is usually provided a clamping signal which determines the level of light intensity to be displayed on the picture tube in response to the video information signal. Values of light intensity during the video information portions of the signal are interpreted by the receiver by reference to the clamping signal level. The composite signal may include other information during the blanking interval, such as tonal pulses, for regulating the color in a color television transmission. These signal portions are not relevant to the invention.
The present invention relates to systems for simulating a graphic arts process. In a graphic arts process, an original color image, such as a color transparency, is used in conjunction with filters to provide a set of color separations, which are black and white transparencies representative of a particular elemental color content in a composite color picture. In the usual graphic arts process, the color separations are used in the etching of color printing plates. A color print is manufactured by sequentially printing three or four different color inks on the same piece of paper using printing plates made from the color separations.
The various steps in the color printing process include adjustments for the regulation of the color content of the eventual print. These adjustments are usually made by a skilled operator who must make adjustments according to his judgement to come out with an acceptable color print. In many cases, the process must be repeated after printing plates have been made and a "proof" made with the plates. The operator on viewing the proof again must use his own judgement to determine what adjustments to the process are required and must then go through the process of modifying the color separations or printing plates in order to obtain an acceptable color print.
In order to aid in the process of making the printing plates and to attempt to eliminate the need for multiple proof printing, there have been developed prior art systems for simulating the color printing process. Such prior systems are disclosed in U.S. Pat. Nos. 3,123,666 to Bailey; 3,128,333 to Loughlin; 3,131,253 to Farber et al.; and 3,800,071 to Reeber, all of which are assigned to the same assignee as the present invention. These prior patents disclose previewing systems which simulate either all, or a portion of, the color printing process and provide a display representative of a color print. The present invention is particularly useful in systems of this type.
In a color printing process previewing system, it is particularly important that there be obtained a very accurate video signal which is representative of the original image. Unlike conventional color TV which uses separate signals for brightness (i.e. black and white) and color information (with reduced bandwidth), previewing systems usually use at least three separate video signals, each representing the elemental color content of the original image. This results in a higher quality color image simulation and display. The three video signals may be derived by the use of a vidicon tube, according to the method taught by Farber et al., or may be derived by using a scanning light source and a photodetector tube. In either case, it is desirable to maximize the range of light values which can be detected and accurately converted into a video signal. In particular, vidicon tubes generally have a limited dynamic range and, therefore, it is important to adjust the amount of light incident on the vidicon tube to an amount which will make maximum use of the tube's dynamic range. In a television system, the light level incident on a vidicon can be arbitrarily adjusted by the operator to a level which gives the picture brightness which the operator considers desirable. In a graphic previewing system such an adjustment of the light level results in a loss of reference with respect to the original brightness of the image being previewed. In such systems, it is important to preserve the original light intensity reference level accurately so that the system displays an accurate simulation of the actual graphic arts process. The invention is equally applicable to other systems wherein it is desired to adjust the amplitude of a signal supplied to a signal transforming apparatus, and then adjust the transforming apparatus output signal to a level representative of the original amplitude of the supplied signal. Thus, the transforming apparatus can operate within its preferred dynamic range.
It is therefore an object of the present invention to provide a signal processing apparatus wherein the level of a supplied signal into a signal transforming apparatus can be adjusted in order to use the optimum dynamic range of the transforming apparatus.
It is a further object of the invention to provide such an apparatus wherein the output signal is representative of the amplitude of the supplied signal.
It is still a further object of the invention to provide such an apparatus wherein the level adjusting mechanism can be automatically returned to its original reference position.